PETER SINGER

ETHICS AND INTUITIONS

(Received 25 January 2005; accepted 26 January 2005)

ABSTRACT. For millennia, philosophers have speculated about the origins of
ethics. Recent research in evolutionary psychology and the neurosciences has shed
light on that question. But this research also has normative significance. A standard
way of arguing against a normative ethical theory is to show that in some circum-
stances the theory leads to judgments that are contrary to our common moral
intuitions. If, however, these moral intuitions are the biological residue of our evo-
lutionary history, it is not clear why we should regard them as having any normative
force. Research in the neurosciences should therefore lead us to reconsider the role of
intuitions in normative ethics.

KEY WORDS: brain imaging, David Hume, ethics, evolutionary psychology,
Henry Sidgwick, Immanuel Kant, intuitions, James Rachels, John Rawls, Jonathan
Haidt, Joshua D. Greene, neuroscience, trolley problem, utilitarianism

1. I

NTRODUCTION

In one of his many fine essays, Jim Rachels criticized philosophers
who ‘‘shoot from the hip.’’ As he put it:

The telephone rings, and a reporter rattles off a few ‘‘facts’’ about something
somebody is supposed to have done. Ethical issues are involved – something
alarming is said to have taken place – and so the ‘‘ethicist’’ is asked for a com-
ment to be included in the next day’s story, which may be the first report the
public will have seen about the events in question.

1

In these circumstances, Rachels noted, the reporters want a short
pithy quote, preferably one that says that the events described are
bad. The philosopher makes a snap judgment, and the result is

The Journal of Ethics (2005) 9: 331–352

Ó Springer 2005

DOI 10.1007/s10892-005-3508-y

1

James Rachels, ‘‘When Philosophers Shoot from the Hip,’’ in Helga Kuhse and

Peter Singer (eds.), Bioethics: An Anthology (Oxford: Blackwell Publishers, 1999),
p. 573.

something that reflects not ‘‘careful analysis’’ but ‘‘accepted wis-
dom.’’ Philosophers become ‘‘orthodoxy’s most sophisticated defend-
ers, assuming that the existing social consensus must be right, and
articulating its theoretical ‘justification’.’’ In contrast, Rachels
argued, philosophers ought to ‘‘challenge the prevailing orthodoxy,
calling into question the assumptions that people unthinkingly
make.’’

2

Rachels’ own work in ethics lived up to that precept. To give just

one of many possible examples, in what is probably his most cited
article, on ‘‘Active and Passive Euthanasia,’’ he set out to criticize the
common intuition that killing is worse than letting die. He showed
that this distinction is influential in medicine, and is embodied in a
statement from the American Medical Association. Then he con-
vincingly argued that this is not an intuition on which we should
rely.

3

In both the papers I have mentioned, Rachels rejected the idea

that the role of moral philosophers is to take our common moral
intuitions as data, and seek to develop the theory that best fits those
intuitions. On the contrary, he maintains, we should be ready to
challenge the intuitions that first come to mind when we are asked
about a moral issue. That is a view that I share, and one I have
written about on several occasions over the years.

4

In the following

pages I argue that recent research in neuroscience gives us new and
powerful reasons for taking a critical stance toward common
intuitions. But I will begin by placing this research in the context
of our long search for the origins and nature of morality.

In the Louvre Museum in Paris there is a black Babylonian column
with a relief showing the sun god Shamash presenting the code of
laws to Hammurabi. Such mythical accounts, bestowing a divine
origin on morality, are common. In Plato’s Protagoras there is an
avowedly mythical account of how Zeus took pity on the hapless
humans, who, living in small groups and with inadequate teeth, weak
claws, and lack of speed, were no match for the other beasts. To make

2

Rachels, ‘‘When Philosophers Shoot from the Hip,’’ p. 575.

3

James Rachels, ‘‘Active and Passive Euthanasia,’’ in Helga Kuhse and Peter

Singer (eds.), Bioethics: An Anthology (Oxford: Blackwell Publishers 1999), pp. 227–
230.

4

Starting with Peter Singer, ‘‘Sidgwick and Reflective Equilibrium,’’ The Monist

58 (1974), pp. 490–517.

PETER SINGER

332

up for these deficiencies, Zeus gave humans a moral sense and the
capacity for law and justice, so that they could live in larger
communities and cooperate with one another. The biblical account of
God giving the Ten Commandments to Moses on Mount Sinai is, of
course, another example.

In addition to these mythical accounts, for at least 2500 years, and

in different civilizations, philosophers have discussed and written
about the nature of ethics. Plato himself was evidently not content
with the account he offered in Protagoras, for in his dialogues he
discusses several other possibilities. In the Republic alone, we have
Thrasymachus’s skeptical claim that the strong, acting in their own
interests, impose morality on the weak, Glaucon’s social contract
model, and Socrates’ proto-natural law defense of justice as the
outcome of a harmony of the different parts of human nature.

Among the questions philosophers have considered are: whether

ethics is objectively true, or relative to culture, or entirely
subjective; whether human beings are naturally good; and whether
ethics comes from nature or from culture. They have regarded such
questions as having a practical, as well as theoretical, significance.
Getting the answers right, they believe, will enable us to live in a
better way.

Many of these thinkers were skilled observers of their fellow

human beings, as well as being among the wisest people of their
times. Consider, for example, the work of Mencius, Aristotle, Niccolo
Machiavelli, Thomas Hobbes, and David Hume. There are many
things about human nature that they understood very well. But none
of them had the advantage of a modern scientific approach to these
issues. Today we have that advantage. Hence it would seem odd if we
could not improve on what they wrote.

In what follows, I summarize some of the new knowledge of ethics

we now possess, knowledge that was not available to any of the great
philosophers I have listed. Then I will consider what normative
significance this new knowledge has. What, if anything, should it
contribute to our debate over how we ought to act?

2. E

VOLUTIONARY

T

HEORY AND THE

O

RIGINS OF

M

ORALITY

The single most important advantage we have over the great moral
philosophers of the past is our understanding of evolution and its
application to ethics. Although the philosophers I have mentioned

ETHICS AND INTUITIONS

333

were able to free themselves from the myth of the divine origin of
morality and to explain morality in naturalistic terms, they lacked a
proper understanding of how our norms may have arisen by natural
selection with the gene as the basic unit for the transmission of
inherited characteristics between generations. Without this knowl-
edge, they could observe our feelings and attitudes but not explain
them adequately. To see what evolutionary theory can add to even
the greatest of the pre-Darwinian thinkers who have speculated about
the origins of morality, consider Hume’s discussion of morality in his
justly celebrated Treatise of Human Nature.

Hume opens his discussion of justice by asking the question

whether justice is a natural or an artificial virtue. In discussing that
question he writes:

A man naturally loves his children better than his nephews, his nephews better
than his cousins, his cousins better than strangers, where every thing else is equal.
Hence arise our common measures of duty, in preferring the one to the other. Our
sense of duty always follows the common and natural course of our passions.

5

Hume gets very close to an evolutionary understanding of the

common sense of duty, but he could not explain, as modern
evolutionary theory can, why ‘‘the common and natural course of
our passions’’ takes the form it does. We now understand that the
genes that lead to the forms of love Hume describes are more likely to
survive and spread among social mammals than genes that do not
lead to preferences for one’s relatives that are typically proportional
to the proximity of the relationship. For we share more genes with
our children than with our cousins, and more with our cousins than
with strangers.

We can also now provide a deeper explanation of the truth of

Hume’s converse, and more controversial, observation that ‘‘there is
no such passion in human minds as the love of mankind, merely as
such, independent of personal qualities, of services, or of relation to
ourself.’’

6

Much as we may regret it, most human beings lack a

general feeling of benevolence for the strangers we pass in the street.
In evolutionary terms, when we consider the species as a whole, the
unit of selection is too large for natural selection to have much
impact. Despite the picture books we had as children, early human

5

David Hume, A Treatise of Human Nature, L. A. Selby-Bigge (ed.) (Oxford:

Clarendon Press, 1978), Book III, Part 2, Section i.

6

Hume, A Treatise of Human Nature, Book III, Part 2, Section i.

PETER SINGER

334

life was not, by and large, a struggle for survival between humans and
sabre-tooth tigers. It was much more often a struggle for survival
between different human beings. There is no evolutionary advantage
in concern for others simply because they are members of our species.
In contrast to the selection of individual organisms within the species,
which is going on all the time, selection between different species
happens too slowly and too rarely to play much of a role in evolution.

Note, however, the factors that Hume lists as generating love for

others: personal qualities, services, and relation to oneself. Related-
ness we have already discussed. Personal qualities may generate
positive feelings because they are likely to be of benefit to us, or to a
small group to which we belong. In contrast to selection between
species, which is rare and of little importance in evolution, selection
within the species, between smaller, isolated breeding groups,
happens much more often. These smaller groups do compete with
each other and, in comparison with species, are relatively short-lived.
The countervailing pressures of selection at the level of the individual
or the gene would still apply, but less effectively. In some circum-
stances, there could be selective pressures that favor self-sacrifice for
the benefit of the group. There would also, of course, be counter-
vailing pressures favoring self-interested actions that do not benefit
the group. If, however, the group develops a culture that rewards
those who risk their own interests in order to benefit the group, and
punishes those who do not, the cost-benefit ratio would be tilted so as
to make benefiting the group more likely to be compatible with
leaving offspring in the next generation.

The third exception that Hume mentioned was ‘‘services.’’ Here

again he touches upon a focus of recent evolutionary theory, which
has meshed with game theory in exploring such situations as the
Prisoners’ Dilemma. This work enables us to give a fuller and more
persuasive answer than Hume could to the question with which he
began his discussion of justice.

Hume asked whether justice is a natural or an artificial virtue, and

answered that it is an artificial one. By that he meant that ‘‘the sense
of justice and injustice is not derived from nature, but arises
artificially, though necessarily from education, and human conven-
tions.’’ He adds that though the rules of justice are artificial, this does
not mean that they are arbitrary. Justice is, for Hume, a human
invention, though one that is ‘‘obvious and absolutely necessary.’’

7

7

Hume, A Treatise of Human Nature, Book III, Part 2, Section i.

ETHICS AND INTUITIONS

335

But justice is not, at least not in its origins, a human invention. We
can find forms of it in our closer nonhuman relatives. A monkey will
present its back to another monkey, who will pick out parasites; after
a time the roles will be reversed. A monkey that fails to return the
favor is likely to be attacked, or scorned in the future. Such
reciprocity will pay off, in evolutionary terms, as long as the costs of
helping are less than the benefits of being helped and as long as
animals will not gain in the long run by ‘‘cheating’’ – that is to say, by
receiving favors without returning them. It would seem that the best
way to ensure that those who cheat do not prosper is for animals to
be able to recognize cheats and refuse them the benefits of
cooperation the next time around. This is only possible among
intelligent animals living in small, stable groups over a long period of
time. Evidence supports this conclusion: reciprocal behavior has been
observed in birds and mammals, the clearest cases occurring among
wolves, wild dogs, dolphins, monkeys, and apes.

Many features of human morality could have grown out of simple

reciprocal practices such as the mutual removal of parasites from
awkward places. Suppose I want to have the lice in my hair picked
out and I am willing in return to remove lice from someone else’s
hair. I must, however, choose my partner carefully. If I help everyone
indiscriminately, I will find myself delousing others without getting
my own lice removed. To avoid this, I must learn to distinguish
between those who return favors and those who do not. In making
this distinction, I am separating reciprocators and nonreciprocators
and, in the process, developing crude notions of fairness and of
cheating. I will strengthen my links with those who reciprocate, and
bonds of friendship and loyalty, with a consequent sense of
obligation to assist, will result.

This is not all. As we see with monkeys, reciprocators are likely to

react in a hostile and angry way to those who do not reciprocate.
More sophisticated reciprocators, able to think and use language,
may regard reciprocity as good and ‘‘right’’ and cheating as bad and
‘‘wrong.’’ From here it is a small step to concluding that the worst of
the nonreciprocators should be driven out of society or else punished
in some way, so that they will not take advantage of others again.
Thus a system of punishment and a notion of desert constitute the
other side of reciprocal altruism.

So Hume was not entirely wrong to say that justice is an artificial

virtue, but he was not entirely right either. The basic rule of
reciprocity, which includes the ability to detect cheats and the sense of

PETER SINGER

336

indignation required to exclude them, is natural in the sense that it
has evolved, is part of our biological nature, and is something we
share with our closer nonhuman relatives. But the more detailed rules
of justice typical of human, language-using societies are refinements
on the instinctive sense of reciprocity, and so may be considered
artificial.

Our biology does not prescribe the specific forms our morality

takes. There are cultural variations in human morality, as even
Herodotus knew.

8

Nevertheless, it seems likely that all these different

forms are the outgrowth of behavior that exists in social animals, and
is the result of the usual evolutionary processes of natural selection.
Morality is a natural phenomenon. No myths are required to explain
its existence.

3. H

OW

H

UMANS

M

AKE

M

ORAL

J

UDGMENTS

Against this background understanding of the origins of morality, I
turn to some recent scientific research that helps us to understand
more specific moral decisions and behavior. To explore the way in
which people reach moral judgments, Jonathan Haidt, a psychologist
at the University of Virginia, asked people to respond to the
following story:

Julie and Mark are brother and sister. They are travelling together in France on
summer vacation from college. One night they are staying alone in a cabin near
the beach. They decided that it would be interesting and fun if they tried making
love. At the very least it would be a new experience for each of them. Julie was al-
ready taking birth control pills, but Mark uses a condom too, just to be safe. They
both enjoy making love but decide not to do it again. They keep that night as a
special secret between them, which makes them feel even closer to each other.
What do you think about that, was it OK for them to make love?

Haidt reports that most people are quick to say that what Julie and
Mark did was wrong. They then try to give reasons for their answer.
They may mention the dangers of inbreeding, but then recall Julie

8

See his account of the efforts of Darius, the Persian Emperor, to persuade people

of different cultures to change their customs in respect of how to dispose of the dead,
in Herodotus, The Histories, Robin Waterfield (trans.) (Oxford: Oxford University
Press, 1998), Book III, Chapter 38.

ETHICS AND INTUITIONS

337

and Mark used two forms of birth control. Or they may suggest that
the siblings could be hurt, even though it is clear from the story that
they were not. Eventually, many people say something like: ‘‘I don’t
know, I can’t explain it, I just know it’s wrong.’’

9

Evidently, it is the

intuitive response that is responsible for the judgment these people
reach, not the reasons they offer, for they stick to their immediate,
intuitive judgment, even after they have withdrawn the reasons they
initially offered for that judgment, and are unable to find better ones.

One example on its own would not show much, but Haidt has

assembled an impressive body of evidence for the view that moral
judgments in a variety of areas are typically the outcome of quick,
almost automatic, intuitive responses. Where there is more deliberate,
conscious reasoning, it tends to come after the intuitive response, and
to be a rationalization of that response, rather than the basis for the
moral judgment.

10

If we turn to our growing knowledge of the parts of the brain

involved in ethical decisions, we find a picture that is consistent with
the conclusions that Haidt has drawn from studies of human
behavior. Here we can begin with Antonio Damasio’s revealing
discussion of the nineteenth century case of Phineas Gage.

11

Gage

was working on the United States railroad when an explosion caused
a 3-ft long iron rod to pass right through his brain. Astonishingly,
Gage survived, and appeared to make a complete recovery, with no
impairment to his reasoning or linguistic abilities. Yet it gradually
became apparent that his character, previously steady and industri-
ous, had changed. He became anti-social, and could not hold down a
steady job as he had before.

Gage’s injury was to the ventromedial portion of the frontal lobes.

More recent patients with damage to this area show the same

9

Jonathan Haidt, Fredrik Bjo¨rklund, and Scott Murphy, ‘‘Moral Dumbfound-

ing: When Intuition Finds No Reason’’ (Department of Psychology, University of
Virginia, 2000, unpublished manuscript); and see further discussion in Jonathan
Haidt, ‘‘The Emotional Dog and Its Rational Tail: A Social Intuitionist Approach to
Moral Judgment,’’ Psychological Review 108 (2001), pp. 814–834. I am indebted to
Joshua Greene for drawing my attention to this, and other material discussed in this
section, which draws on Joshua Greene, The Terrible, Horrible, No Good, Very Bad
Truth About Morality, and What to Do About It

(Ph.D. dissertation, Department of

Philosophy, Princeton University, 2002), Chapter 3.

10

Haidt, ‘‘The Emotional Dog and Its Rational Tail: A Social Intuitionist Ap-

proach to Moral Judgment.’’

11

Antonio R. Damasio, Descartes’ Error: Emotion, Reason, and the Human Brain

(New York: Grosset/Putnam, 1994), pp. 3–9, 34–51.

PETER SINGER

338

combination of intact reasoning abilities but increased breaches of
the usual moral and social standards. These patients appear to be
emotionally deficient, not reacting in the usual way to gory scenes in
which people’s lives were lost or endangered. Damasio says of one of
them that his predicament was ‘‘To know, but not to feel.’’

12

Brain-

imaging studies have found a correlation between anti-social behav-
ior and a deficiency in either the size of, or the amount of metabolic
activity in, the prefrontal cortex.

13

In two patients where the damage

to the ventromedial portion of the front lobes occurred early in life,
the patients had much more marked psychopathic tendencies. They
lied, stole and acted violently, and lacked any remorse.

14

Further insight into the way in which we make moral judgments

has come very recently from experiments using functional Magnetic
Resonance Imaging, or fMRI, conducted by Joshua Greene and
others at Princeton University. Greene designed the experiments to
throw light on the way in which people respond to situations known
in the philosophical literature as ‘‘trolley problems.’’

15

In the

standard trolley problem, you are standing by a railroad track when
you notice that a trolley, with no one aboard, is rolling down the
track, heading for a group of five people. They will all be killed if the
trolley continues on its present track. The only thing you can do to
prevent these five deaths is to throw a switch that will divert the
trolley onto a side track, where it will kill only one person. When
asked what you should do in these circumstances, most people say

12

Damasio, Descartes Error: Emotion, Reason, and the Human Brain, p. 45.

13

Adrian Raine, Todd Lencz, Susan Bihrle, Lori LaCasse, and Patrick Colletti,

‘‘Reduced Prefrontal Gray Matter Volume and Reduced Autonomic Activity in
Antisocial Personality Disorder,’’ Archives of General Psychiatry 57 (2000), pp. 119–
127; Adrian Raine, Monte S. Buchsbaum, Jill Stanley, Steven Lottenberg, Leonard
Abel and Jacqueline Stoddard, ‘‘Selective Reductions in Prefrontal Glucose
Metabolism in Murderers,’’ Biological Psychiatry 36 (1994), pp. 365–373.

14

Steven W. Anderson, Antoine Bechara, Hanna Damasio’ Daniel Tranel and

Antonio R. Damasio, ‘‘Impairment of Social and Moral Behavior Related to Early
Damage in Human Prefrontal Cortex,’’ Nature Neuroscience 2 (1999), pp. 1032–
1037.

15

Phillipa Foot appears to have been the first philosopher to discuss these

problems, in Phillipa Foot, ‘‘The Problem of Abortion and the Doctrine of the
Double Effect,’’ Oxford Review 5 (1967), pp. 5–15; reprinted in James Rachels (ed.),
Moral Problems: A Collection of Philosophical Essays

(New York: Harper& Row,

1971), pp. 28–41. The classic article on the topic, however, is Judith Jarvis Thomson,
‘‘Killing, Letting Die, and the Trolley Problem,’’ The Monist 59 (1976): 204–217.

ETHICS AND INTUITIONS

339

that you should divert the trolley onto the side track, thus saving four
lives.

In another version of the problem, the trolley, as before, is about

to kill five people. This time, however, you are not standing near the
track, but on a footbridge above the track. You cannot divert the
trolley. You consider jumping off the bridge, in front of the trolley,
thus sacrificing yourself to save the imperiled people, but you realize
that you are far too light to stop the trolley. Standing next to you,
however, is a very large stranger. The only way you can stop the
trolley killing five people is by pushing this large stranger off the
footbridge, in front of the trolley. If you push the stranger off, he will
be killed, but you will save the other five. When asked what you
should do in these circumstances, most people say that you should
not push the stranger off the bridge.

Many philosophers, including Judith Jarvis Thomson, see the

problem posed by this pair of cases like this. In both cases you bring
about the death of one person to save five, but we judge your action
as right in the standard trolley case, and as wrong in the footbridge
case. What is it that makes the difference between these two cases?
These philosophers thus take the moral intuitions elicited by the cases
as correct, and seek to justify them. But every time a seemingly
plausible justifying principle has been suggested, other philosophers
have produced variants on the original pair of cases that show that
the suggested principle does not succeed in justifying our intuitive
responses. For example, some philosophers suggested that the
difference between the standard trolley case and the footbridge case
is that in the latter the stranger is used as a means to save the others.
Thus pushing the stranger off the footbridge violates the Kantian
injunction not to use another person merely as a means, while
throwing the switch does not. Unfortunately for proponents of this
neat explanation, we can imagine a case in which throwing the switch
does not cause the trolley to run down an altogether different track,
but makes it go around a loop before it reaches the five people
threatened by it. On that loop, the very large stranger is lying.
Because he is so large, his body will bring the trolley to a stop, but not
before it kills him. To divert the trolley around this loop does use the
stranger as a means to saving the life of the other five, but most
people consider it would be right to do it. They thus judge this case as
closer to the standard case of throwing the switch than to the case of
pushing the stranger off the footbridge.

PETER SINGER

340

Unlike the many philosophers who have tried to justify our

intuitions in these situations, Greene was more concerned to
understand why we have them. He thought that the roots of the
differing judgments we make about the two situations may lie in our
different emotional responses to the idea of causing a stranger’s death
by throwing a switch on a railway track, and pushing someone to his
or her death with our bare hands. As Greene puts it:

Because people have a robust, negative emotional response to the personal viola-
tion proposed in the footbridge case they immediately say that it’s wrong... At the
same time, people fail to have a strong negative emotional response to the rela-
tively impersonal violation proposed in the original trolley case, and therefore re-
vert to the most obvious moral principle, ‘‘minimize harm,’’ which in turn leads
them to say that the action in the original case is permissible.

16

Greene used fMRI imaging, which provides a real-time image of
activity in different parts of the brain, to test this hypothesis. He
predicted that people asked to make a moral judgment about
‘‘personal’’ violations like pushing the stranger off the footbridge
would show increased activity in areas of the brain associated with
the emotions, when compared with people asked to make judgments
about relatively ‘‘impersonal’’ violations like throwing a switch. But
he also made a more specific prediction: that the minority of subjects
who do consider that it would be right to push the stranger off the
footbridge would, unless they were psychopaths, be giving this
response in spite of their emotions, and therefore they would take
longer to reach this judgment than those who say that it would be
wrong to push the stranger off the footbridge, and also longer than
they would take to reach a judgment in a case that did not arouse
such strong emotional responses.

Greene’s predictions were confirmed. When people were asked to

make judgments in the ‘‘personal’’ cases, parts of their brains
associated with emotional activity were more active than when they
were asked to make judgments in ‘‘impersonal’’ cases. More
significantly, those who came to the conclusion that it would be
right to act in ways that involve a personal violation, but minimize
harm overall – for example, those who say that it would be right to

16

Greene, The Terrible, Horrible, No Good, Very Bad Truth About Morality, and

What to Do About It

, p. 178.

ETHICS AND INTUITIONS

341

push the stranger off the footbridge – took longer to form their
judgment than those who said it would be wrong to do so.

17

When Greene looked more closely at the brain activity of these

subjects who say ‘‘yes’’ to personal violations that minimize overall
harm, he found that they show more activity in parts of the brain
associated with cognitive activity than those who say ‘‘no’’ to such
actions.

18

These are preliminary results, based on a limited amount

of data. But let us assume that they are sound, and speculate on
what might follow from them, in conjunction with the other
scientific information relevant to the origins of ethics, as outlined
above.

4. N

ORMATIVE

I

MPLICATIONS

Shortly after The Origin of Species appeared, Darwin wrote to a
friend: ‘‘I have received in a Manchester newspaper rather a good
squib, showing that I have proved ‘might is right’...’’

19

Darwin knew,

of course, that he had done nothing of the sort. The Social Darwinists
committed the same fallacy when they argued against state interfer-
ence with the free market on the grounds that protecting the poor and
weak was interfering with natural selection. Assuming that we can
define the term ‘‘natural’’ in a way that makes it meaningful to say
that protecting the poor and weak interferes with natural selection,
we would still need an ethical argument to say that it is wrong to do
so. The direction of evolution neither follows, nor has any necessary
connection with, the path of moral progress. ‘‘More evolved’’ does
not mean ‘‘better.’’ No matter how often the fallacy of reading a

17

Joshua D. Greene, R. Brian Sommerville, Leigh E. Nystrom, John M. Darley

and onathan D. Cohen, ‘‘An fMRI Investigation of Emotional Engagement in
Moral Judgment,’’ Science 293 (2001), pp. 2105–2108. To be more specific: in per-
sonal moral dilemmas, the medial frontal cortex, posterior cingulate cortex, and
angular gyrus/superior temporal sulcus are active. In impersonal moral dilemmas
there is increased activity in the dorsolateral prefrontal cortex and parietal lobe.

18

Joshua Greene and Jonathan Haidt, ‘‘How (and Where) Does Moral Judgment

Work?’’ Trends in Cognitive Sciences 6 (2002), pp. 517–523, and personal commu-
nications. To be more specific, those who accept the personal violation show more
anterior dorsolateral prefrontal activity while those who reject it have more activity
in the precuneus area.

19

Darwin to Charles Lyell, in Francis Darwin (ed.), The Life and Letters of

Charles Darwin

, Volume II (London: Murray, 1887), p. 262.

PETER SINGER

342

moral direction into evolution has been pointed out, people still
commit it, and it is not difficult to find otherwise excellent
contemporary writers in evolutionary theory who continue to make
this mistake. Nevertheless, it is a mistake.

20

So while I have claimed

that evolutionary theory explains much of common morality,
including the central role of duties to our kin, and of duties related
to reciprocity, I do not claim that this justifies these elements of
common morality. I am a supporter of an evolutionary approach to
human behavior, and I am interested in ethics, but I am not an
advocate of an ‘‘evolutionary ethic.’’

The impossibility of deducing ethical conclusions from the facts of

evolution does not mean that recent advances in our scientific
understanding of ethics have no normative significance at all. These
advances are highly significant for normative ethics, but in an indirect
way. To appreciate this, we need to look at the current debate over
methodology in normative ethics.

A dominant theme in normative ethics for the past century or more

has been the debate between those who support a systematic
normative ethical theory – utilitarianism and other forms of conse-
quentialism have been the leading contenders – and those who ground
their normative ethics on our common moral judgments or intuitions.
In this debate, the chief weapons of opponents of utilitarianism have
been examples intended to show that the dictates of utilitarianism
clash with moral intuitions that we all share. Perhaps the most famous
literary instance occurs in The Karamazov Brothers, where Dostoyev-
sky has Ivan challenge Alyosha to say whether he would consent to
build a world in which people were happy and at peace, if this ideal
world could be achieved only by torturing ‘‘that same little child
beating her chest with her little fists.’’ Alyosha says that he would not
consent to build such a world on those terms.

21

Hastings Rashdall

thought he could refute hedonistic utilitarianism by arguing that it
cannot explain the value of sexual purity.

22

H. J. McCloskey, writing

20

See, for example, Edward O. Wilson, On Human Nature (Cambridge: Harvard

University Press, 1978), p. 5

21

Fyodor Dostoyevsky, The Karamazov Brothers, Ignat Avsey (trans.) (Oxford:

Oxford University Press, 1994), Part 2, Book 5, Chapter 4.

22

Hastings Rashdall, The Theory of Good and Evil, Volume 1 (Oxford: Clarendon

Press, 1907), p. 197.

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at a time when lynchings in the U.S. South were still a possibility,
thought it a decisive objection to utilitarianism that the theory might
direct a sheriff to frame an innocent man in order to prevent a white
mob lynching half a dozen innocents in revenge for a rape.

23

Bernard

Williams offered a similar example, of a botanist who wanders into a
village in the jungle where 20 innocent people are about to be shot. He
is told that nineteen of them will be spared, if only he will himself
shoot the twentieth. Though Williams himself did not say that it
would necessarily be wrong to shoot the twentieth, he thought that
utilitarianism could not account for the difficulty of the decision.

24

Initially, the use of such examples to appeal to our common moral

intuitions against consequentialist theories was an ad hoc device
lacking metaethical foundations. It was simply a way of saying: ‘‘If
Theory U is true, then in situation X you should do Y. But we know
that it would be wrong to do Y in X, therefore U cannot be true.’’
This is an effective argument against U, as long as the judgment that
it would be wrong to do Y in X is not challenged. But the argument
does nothing to establish that it is wrong to do Y in X, nor what a
sounder theory than U would be like. In A Theory of Justice, John
Rawls took the crucial step towards fusing this argument with an
ethical methodology when he argued that the test of a sound moral
theory is that it can achieve a ‘‘reflective equilibrium’’ with our
considered moral judgments. By ‘‘reflective equilibrium’’ Rawls
meant that, where there is no inherently plausible theory that
perfectly matches our initial moral judgments, we should modify
either the theory, or the judgments, until we have an equilibrium
between the two. The model here is the testing of a scientific theory.
In science, we generally accept the theory that best fits the data, but
sometimes, if the theory is inherently plausible, we may be prepared
to accept it even if it does not fit all the data. We might assume that
the outlying data are erroneous, or that there are still undiscovered
factors at work in that particular situation. In the case of a normative
theory of ethics, Rawls assumes, the raw data is our prior moral
judgments. We try to match them with a plausible theory, but if we
cannot, we reject some of the judgments, and modify the theory so

23

H. J. McCloskey, ‘‘An Examination of Restricted Utilitarianism,’’ in Michael

D. Bayles (ed.), Contemporary Utilitarianism (Gloucester: Peter Smith, 1978), where
the example is on p. 121.

24

Bernard Williams, ‘‘A Critique of Utilitarianism,’’ in J. J. C. Smart and

Bernard Williams, Utilitarianism; For and Against (Cambridge: Cambridge
University Press, 1973), pp. 96–100, 110–117.

PETER SINGER

344

that it matches others. Eventually the plausibility of the theory and of
the surviving judgments reach an equilibrium, and we then have the
best possible theory. On this view the acceptability of a moral theory
is not determined by the internal coherence and plausibility of the
theory itself, but, to a significant extent, by its agreement with those
of our prior moral judgments that we are unwilling to revise or
abandon. In A Theory of Justice Rawls uses this model to justify
tinkering with his original idea of a choice arising from a hypothetical
contract, until he is able to produce results that are not too much at
odds with our ordinary ideas of justice.

25

The model of reflective equilibrium has always struck me as

dubious. The analogy between the role of a normative moral theory
and a scientific theory is fundamentally misconceived.

26

A scientific

theory seeks to explain the existence of data that are about a world
‘‘out there’’ that we are trying to explain. Granted, the data may have
been affected by errors in measurement or interpretation, but unless
we can give some account of what the errors might have been, it is not
up to us to choose or reject the observations. A normative ethical
theory, however, is not trying to explain our common moral
intuitions. It might reject all of them, and still be superior to other
normative theories that better matched our moral judgments. For a
normative moral theory is not an attempt to answer the question
‘‘Why do we think as we do about moral questions?’’ Even without
an evolutionary understanding of ethics, it is obvious that the
question ‘‘Why do we think as we do about moral questions?’’ may
require a historical, rather than a philosophical, investigation. On
abortion, suicide, and voluntary euthanasia, for instance, we may
think as we do because we have grown up in a society that was, for
nearly 2000 years, dominated by the Christian religion. We may no
longer believe in Christianity as a moral authority, but we may find it
difficult to rid ourselves of moral intuitions shaped by our parents
and our teachers, who were either themselves believers, or were
shaped by others who were.

A normative moral theory is an attempt to answer the question

‘‘What ought we to do?’’ It is perfectly possible to answer this

25

John Rawls, A Theory of Justice (Cambridge: Harvard University Press, 1971),

p. 48. The idea of reflective equilibrium was already present in Rawls’s ‘‘Outline of a
Decision Procedure for Ethics,’’ The Philosophical Review 60 (1951), pp. 177–197.
The analogy with a scientific theory is explicit in the earlier article.

26

See Singer, ‘‘Sidgwick and Reflective Equilibrium.’’

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question by saying: ‘‘Ignore all our ordinary moral judgments, and
do what will produce the best consequences.’’ Of course, one would
need to give some kind of argument for this answer. My concern now
is not to give this argument, or any other argument for possible
alternatives to whatever theory best explains our intuitive judgments.
My point is that the model of reflective equilibrium, at least as
presented in A Theory of Justice, appears to rule out such an answer,
because it assumes that our moral intuitions are some kind of data
from which we can learn what we ought to do.

Rawls addressed the metaethical implications of his method

again in Political Liberalism.

27

There he distinguished it from old-

fashioned ethical intuitionism, describing it instead as ‘‘Kantian
constructivism.’’ Whereas intuitionism seeks to defend our intu-
itions as offering rational insight into true ethical principles,
constructivism replaces this by a search for ‘‘reasonable grounds
of reaching agreement rooted in our conception of ourselves and in
our relation to society.’’ We cannot, on this view, discover moral
truth. We can only construct our moral views from concepts and
ideas that we already have.

One evident objection to Rawls’s Kantian constructivism is that it

makes ethics culturally relative. Different peoples, with differing
conceptions of themselves and their relation to society, might
construct different theories that lead them to different principles of
justice. Should that be the case, it could not then be said that one set
of principles is true and the other false. The most that can be claimed
for the particular principles of justice that Rawls defends is that they
offer reasonable grounds of agreement for people holding ‘‘our’’
conception of ourselves and our relation to society. But some may
not see this as an objection. Cultural relativism has had many
defenders in ethics, including many who misguidedly believe that it
offers a defense against cultural imperialism (This is the reverse of the
truth. If ethics is culturally relative, and my culture gives great value
to imposing our values on other cultures, ethical relativism allows no
foothold for arguing that we are mistaken in believing that it is good
to impose our values on others). I do not, however, want to dwell on
the relativist element of Kantian constructivism, because I want to
make a more general objection to any method of doing ethics that
judges a normative theory either entirely, or in part, by the extent to
which it matches our moral intuitions.

27

John Rawls, Political Liberalism (New York: Columbia University Press, 1993).

PETER SINGER

346

Admittedly, it is possible to interpret the model of reflective

equilibrium so that it takes into account any grounds for objecting to
our intuitions, including those that I have put forward. Norman
Daniels has argued persuasively for this ‘‘wide’’ interpretation of
reflective equilibrium.

28

If the interpretation is truly wide enough to

countenance the rejection of all our ordinary moral beliefs, then I
have no objection to it. The price for avoiding the inbuilt conser-
vatism of the narrow interpretation, however, is that reflective
equilibrium ceases to be a distinctive method of doing normative
ethics. Where previously there was a contrast between the method of
reflective equilibrium and ‘‘foundationalist’’ attempts to build an
ethical system outward from some indubitable starting point, now
foundationalism simply becomes the limiting case of a wide reflective
equilibrium.

Let us return for a moment to the trolley problem cases. As

mentioned before, philosophical discussions of these cases from
Thomson onwards have been preoccupied with the search for
differences between the cases that justify our initial intuitive
responses. If, however, Greene is right to suggest that our intuitive
responses are due to differences in the emotional pull of situations
that involve bringing about someone’s death in a close-up, personal
way, and bringing about the same person’s death in a way that is at a
distance, and less personal, why should we believe that there is
anything that justifies these responses? If Greene’s initial results are
confirmed by subsequent research, we may ultimately conclude that
he has not only explained, but explained away the philosophical
puzzle (I say that we may ultimately reach this conclusion because of
course Greene’s data alone cannot prove any normative view right or
wrong. Normative argument is needed, of the kind I shall sketch
below, to link those data with a particular normative view).

This becomes clearer when we consider how well Greene’s findings

fit into the broader evolutionary view of the origins of morality
outlined earlier in this paper.

29

For most of our evolutionary history,

human beings have lived in small groups, and the same is almost
certainly true of our pre-human primate and social mammal
ancestors. In these groups, violence could only be inflicted in an
up-close and personal way – by hitting, pushing, strangling, or using

28

See Norman Daniels, Justice and Justification: Reflective Equilibrium in Theory

and Practice

(Cambridge: Cambridge University Press, 1997).

29

As Greene himself has pointed out. See Footnote 8, above.

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347

a stick or stone as a club. To deal with such situations, we have
developed immediate, emotionally based responses to questions
involving close, personal interactions with others. The thought of
pushing the stranger off the footbridge elicits these emotionally based
responses. Throwing a switch that diverts a train that will hit
someone bears no resemblance to anything likely to have happened in
the circumstances in which we and our ancestors lived. Hence the
thought of doing it does not elicit the same emotional response as
pushing someone off a bridge. So the salient feature that explains our
different intuitive judgments concerning the two cases is that the
footbridge case is the kind of situation that was likely to arise during
the eons of time over which we were evolving; whereas the standard
trolley case describes a way of bringing about someone’s death that
has only been possible in the past century or two, a time far too short
to have any impact on our inherited patterns of emotional response.
But what is the moral salience of the fact that I have killed someone
in a way that was possible a million years ago, rather than in a way
that became possible only two hundred years ago? I would answer:
none.

Thus recent scientific advances in our understanding do have some

normative significance, and at different levels. At the particular level
of the analysis of moral problems like those posed by trolley cases, a
better understanding of the nature of our intuitive responses suggests
that there is no point in trying to find moral principles that justify the
differing intuitions to which the various cases give rise. Very
probably, there is no morally relevant distinction between the cases.
At the more general level of method in ethics, this same understand-
ing of how we make moral judgments casts serious doubt on the
method of reflective equilibrium. There is little point in constructing a
moral theory designed to match considered moral judgments that
themselves stem from our evolved responses to the situations in which
we and our ancestors lived during the period of our evolution as
social mammals, primates, and finally, human beings. We should,
with our current powers of reasoning and our rapidly changing
circumstances, be able to do better than that.

A defender of the idea of reflective equilibrium might say that

these arguments against giving weight to certain intuitions can
themselves, on the model of ‘‘wide reflective equilibrium,’’ be part of
the process of achieving equilibrium between a theory and our
considered moral judgments. The arguments would then lead us to
reject judgments that we might otherwise retain, and so end up with a

PETER SINGER

348

different normative theory. As we have already noted, making the
model of ‘‘reflective equilibrium’’ as all-embracing as this may make
it salvageable, but only at the cost of making it close to vacuous. For
with this change, the ‘‘data’’ that a sound moral theory is supposed to
match have become so changeable that they can play, at best, a minor
role in determining the final shape of the normative moral theory.
Finally, for the same reasons that reflective equilibrium no longer
appeals as a way of testing a moral theory, so Kantian constructivism
ceases to be an attractive metaethic, whether it ends up being
culturally relative or not. To the extent that ‘‘our conception of
ourselves’’ is tied up with our intuitive ideas of right and wrong, we
may question why we should be concerned to construct a moral view
out of our evolved intuitions about what is the right way to act in
particular situations. Moreover, a Kantian constructivist who man-
ages to avoid cultural relativism by finding universally shared
intuitive ideas of right and wrong may have shown nothing more
than that our common evolutionary heritage has, unsurprisingly,
given us a common set of intuitive ideas of right and wrong.

What I am saying, in brief, is this. Advances in our understanding

of ethics do not themselves directly imply any normative conclusions,
but they undermine some conceptions of doing ethics which
themselves have normative conclusions. Those conceptions of ethics
tend to be too respectful of our intuitions. Our better understanding
of ethics gives us grounds for being less respectful of them.

5. C

ONCLUSION

: A W

AY

F

ORWARD

?

Whenever it is suggested that normative ethics should disregard our
common moral intuitions, the objection is made that without
intuitions, we can go nowhere. There have been many attempts,
over the centuries, to find proofs of first principles in ethics, but most
philosophers consider that they have all failed. Even a radical ethical
theory like utilitarianism must rest on a fundamental intuition about
what is good. So we appear to be left with our intuitions, and nothing
more. If we reject them all, we must become ethical skeptics or
nihilists.

There are many ways in which one might try to respond to this

objection, and I do not have the time here to review them all. So let
me suggest just one possibility. Haidt’s behavioral research and
Greene’s brain imaging studies suggest the possibility of distinguishing

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349

between our immediate emotionally based responses, and our more
reasoned conclusions. In everyday life, as Haidt points out,
our reasoning is likely to be nothing more than a rationalization
for our intuitive responses – as Haidt puts it, the emotional dog is
wagging the rational tail. But Greene’s research suggests that in some
people, reasoning can overcome an initial intuitive response. That, at
least, seems the most plausible way to account for the longer reaction
times in those subjects who, in the footbridge example, concluded
that you would be justified in pushing the stranger in front of the
trolley. These people appear to have had the same emotional
responses against pushing the stranger, but further thought led them
to reject that emotional response and to give a different answer. The
preliminary data showing greater activity in parts of their brain
associated with cognitive processes suggests the same conclusion.
Moreover, the answer these subjects gave is, surely, the rational
answer. The death of one person is a lesser tragedy than the death of
five people. That reasoning leads us to throw the switch in the
standard trolley case, and it should also lead us to push the stranger
in the footbridge, for there are no morally relevant differences
between the two situations (Although we may decide to withhold our
praise from people who are capable of pushing someone off a
footbridge in these circumstances. As Henry Sidgwick pointed out in
The Methods of Ethics

, it is important to distinguish between the

utility of an action, and the utility of praising or blaming that action.
We may not wish to praise those who are capable of pushing
strangers off high places, for fear that they will do it on other
occasions when it does not save more lives than it costs.

30

)

It might be said that the response that I have called ‘‘more

reasoned’’ is still based on an intuition, for example the intuition that
five deaths are worse than one, or more fundamentally, the intuition
that it is a bad thing if a person is killed. But if this is an intuition, it is
different from the intuitions to which Haidt and Greene refer. It does
not seem to be one that is the outcome of our evolutionary past. We
have already noted Hume’s observation that ‘‘there is no such
passion in human minds as the love of mankind, merely as such’’ and
as we have seen, there is a good evolutionary reason for why this
should be so. Thus the ‘‘intuition’’ that tells us that the death of one
person is a lesser tragedy than the death of five is not like the

30

Henry Sidgwick, The Methods of Ethics, Seventh Edition (London: Macmillan,

1907), pp. 428–429.

PETER SINGER

350

intuitions that tell us we may throw the switch, but not push the
stranger off the footbridge. It may be closer to the truth to say that it
is a rational intuition, something like the three ‘‘ethical axioms’’ or
‘‘intuitive propositions of real clearness and certainty’’ to which
Henry Sidgwick appeals in his defense of utilitarianism in The
Methods of Ethics

. The third of these axioms is ‘‘the good of any one

individual is of no more importance, from the point of view (if I may
say so) of the Universe, than the good of any other.’’

31

Perhaps here, after finding ourselves in broad agreement with

Hume for so much of this paper, we find the need to appeal to
something in Hume’s polar opposite, Immanuel Kant. Kant thought
that unless morality could be based on pure reason, it was a
chimera.

32

Perhaps he was right. In the light of the best scientific

understanding of ethics, we face a choice. We can take the view that
our moral intuitions and judgments are and always will be emotion-
ally based intuitive responses, and reason can do no more than build
the best possible case for a decision already made on nonrational
grounds. That approach leads to a form of moral skepticism,
although one still compatible with advocating our emotionally based
moral values and encouraging clear thinking about them.

33

Alterna-

tively, we might attempt the ambitious task of separating those moral
judgments that we owe to our evolutionary and cultural history, from
those that have a rational basis. This is a large and difficult task. Even
to specify in what sense a moral judgment can have a rational basis is
not easy. Nevertheless, it seems to me worth attempting, for it is the
only way to avoid moral skepticism.

31

Sidgwick, The Methods of Ethics, p. 382.

32

Immanuel Kant, Groundwork of the Metaphysics of Morals, Mary Gregor

(trans.) (Cambridge: Cambridge University Press, 1997), Section II.

33

Greene takes this position in The Terrible, Horrible, No Good, Very Bad Truth

about Morality, and What to Do About It

. He describes his view as moral skepticism,

but distinguishes it from moral nihilism, in which there is no place for moral values
at all. I am grateful to Greene, not only for his illuminating research, but for his
valuable comments on this paper. Other helpful comments have come from people
too numerous to mention individually, so I offer collective thanks to all those who
spoke up when I presented this paper at the James Rachels Memorial Conference at
the University of Alabama, Birmingham; at the Princeton University Center for
Human Values Fellows’ Seminar; and at Philosophy Departments at the following
universities: University of Melbourne, University of Vermont, Rutgers University,
and the University of Lodz. Despite all this advice, I am aware that there is much
more work needed: this paper is no more than a sketch of an argument that I hope to
develop more adequately in future.

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University Center for Human Values
Princeton University
5 Ivy Lane
Princeton, NJ 08544
USA

and

Centre for Applied Philosophy and Public Ethics
University of Melbourne
Melbourne, Victoria 3010
Australia

PETER SINGER

352